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Figure 1. Climate drivers: First-, second-, third-, and fourth-order climate controls.

Climate drivers can be categorized by the dual attributes of range of temperature change forced by the driver and the length of time that the driver cycles through. In this graph, the vertical axis (time) is logarithmic (units are powers of 10), and the horizontal axis (temperature effect/change) is arithmetic. This interpretation permits comparison of climate drivers with their potential effects, and separation of drivers of different magnitude. Note that human intervention, which may or may not exist, is in the same category as some other small natural drivers. Diamond shapes are interpreted and literature-documented ranges of values; dots indicate possible ranges of values. The authors recognize that the error bars for these interpretations are broad, but the phenomena appear to fall into significant categories of effects and time.

First-order climate controls: Earth has a life-supporting climate because of its distance from the sun, solar luminosity, and the evolution of a greenhouse atmosphere of water vapor, methane, CO2, and other gases that trap solar energy and make it usable. This atmosphere evolved over the last 4.5 billion years and continues to evolve. For instance, Berner (1994) suggested that the carbon-dioxide content has decreased over the past 600 million years from 18 times the current concentration (see also Moore et al., 1997, p. 27). The greenhouse effect itself makes the earth 20o–40oC warmer than it would otherwise be (Pekarek, Chapter 1, GPGCC, 2001; Moore et al., 1997, p. 10, 12).

Second-order climate controls: Distribution of oceans and continents on the surface of the earth controls ocean currents, which distribute heat. This fundamental concept (Gerhard and Harrison, Chapter 2, GPGCC, 2001) explains the 15o–20oC climate variations over hundreds of million of years (Lang et al., 1999; Frakes, 1979, p. 203). Such variations are exemplified by the two major earth cycles between glacial “icehouse” and warm “greenhouse” states. The late Precambrian “icehouse” evolved into the Devonian “greenhouse,” then the Carboniferous “icehouse,” then the Cretaceous “greenhouse,” which evolved to the present “icehouse” state. Redistribution of heat around the earth is determined by the presence of equatorial currents that keep and thrust warm water masses away from the poles. Blockage of such currents, which permits the formation of gyres that move warm waters to the poles, creates the setting that allows continental-scale glaciation.

Third-order climate controls: Solar insolation variability has emerged as a major climate driver, as are the orbital variations that change the distance between the earth and the sun (Hoyt and Schatten, 1997; Frakes, 1979, p. 9; Pekarek, Chapter 1, GPGCC, 2001; Fischer, 1982; Berger et al., 1984). In addition, large-scale changes in ocean circulation through changes in current structure can be significant climate drivers (Broecker, 1997, 1999). Large-scale ocean tidal cycles may drive climate, including the large-scale maximum and minimum associated with the Medieval Climate Optimum and the Little Ice Age, on an 1800-year cycle with a 5000-year modulation (Keeling and Whorf, 2000). These drivers may cause temperature changes of 5o–15oC over hundreds to hundreds of thousands of years.

Fourth-order climate controls: There are many drivers that control small temperature changes (up to 5oC) over short periods of time (up to hundreds of years). Many are natural phenomena, including smaller-scale oceanographic oscillations (La Niña and El Niño), volcanic activity (such as the eruptions of Pinatubo and Krakatoa) (Moore et al., 1997, p. 47), solar storms and flares (Hoyt and Schatten, 1997, p. 168, 198, 199), small orbital changes (Frakes, 1979; NASA Web site), meteorite impacts, and human intervention (such as human-derived carbon dioxide [CO2] and methane [CH4] alterations to atmospheric composition). Tectonic and topographic uplift have small temperature effects and are regional rather than global, but may be of long duration (Ruddiman, 1997, p. 178, 502). Eighteen-, 90-, and 180-year cycles driven by ocean tides have been recognized by Keeling and Whorf (1997), and they drive climate by changing heat transfer rates between oceans and atmosphere.